The invention relates to a method for determining indirectly the concentration of a predetermined substance in the blood by measuring the concentration of said substance and the concentration of water vapour in a person""s exhalation air and utilizing a known relationship between these concentrations.
The method according to the invention is proposed particularly for determining alcohol concentration but can be applied also for determining the concentration of other substances which may occur in the blood.
U.S. Pat. No. 4,314,564 describes such a method for determining alcohol concentration and also an apparatus for working the method, exhalation taking place directly into a tube from one end thereof via a mouthpiece. When one and the same apparatus shall be used for performing sequential alcohol tests on several different persons the mouthpiece must be exchangeable for hygienic reasons so there is used a disposable mouthpiece for each person being tested, which is an economic burden when using the apparatus for mass tests the mouthpiece exchange at each test also requiring attendance and making performance of the test more lengthy. Moreover, the necessity of the person to be tested placing the mouthpiece against the lips means that the test hardly can be performed without attracting public attention, which in specific situations may be rather annoying to the person being tested.
U.S. Pat. No. 5,376,555 describes method and device for determining the concentration of alveolar alcohol in exhalation air blown into an infrared sensing device. The presence of alcohol from the respiratory tract of the person being tested is detected by continuously monitoring alcohol and carbon dioxide, normalizing alcohol values with respect to carbon dioxide, calculating a difference between normalized alcohol concentration and carbon dioxide concentration over time, integrating the difference, and comparing the integrated difference with a threshold value. The alcohol concentration in the blood cannot be determined in a reliable manner by this method because the carbon dioxide content in the exhalation air is not constant but varies from one breath to another and also during the breath.
In the most common method according to prior art technique for determining the alcohol concentration in the blood of a person, for example in a traffic control a breathing test is performed wherein the alcohol concentration at the end of a deep exhalation is measured. In that case the maximum alcohol concentration in the alveolar air (that part of the exhalation air which has been deepest in the lung) is measured. Since the exact temperature in the lung is unknown and accordingly also the temperature at which the alcohol has been evaporated is unknown there will be an error in the determination of the blood alcohol content, which amounts to about 7%/xc2x0 C. deviation. The lung temperature may vary several degrees in dependence of the body temperature (fever), the outside temperature, breathing pattern, body strain etc., and the error may be considerable. By measuring the temperature of the exhaled alveolar air one can partly compensate for this error but since the temperature recrease from the lung to the point of measuring will be great and will vary with breathing pattern, the outside temperature etc. the remaining error also in this case will be considerable.
U.S. Pat. No. 5,458,853 describes a device for analyzing a breath sample particularly for determining if a driver is under the influence of alcohol or not. The device allows sampling by two different methods: in one sampling method the test person is exhaling towards an inlet port, and in the other sampling method the person is exhaling through a mouthpiece. The first-mentioned sampling method constitutes xe2x80x9cpassivexe2x80x9d sampling and is effected very discretely without manual operations. By this sampling those persons can be screened off who have no alcohol at all in the blood. These persons undergo no further sampling while the persons who turn out to have alcohol in the blood for a more accurate determination of the alcohol content undergo sampling according to the other method which requires use of a mouthpiece which for hygienic reasons must be exchanged after each sampling. In the xe2x80x9cpassivexe2x80x9d sampling the exhalation can comprise an entirely common exhalation so that measuring can be effected on persons who exhale freely and entirely normally. In other words, no special breathing technique is required for performing the test. As a consequence thereof the test can be performed very discretely without the test being noticed by people closest to the tested person or even by this person himself. Accordingly a screening test on several persons can be sequentially effected without the necessity of taking manual measures at each individual test such as exchange of a mouthpiece or another article of consumption. The sampling is performed in a hygienic way and moreover in a way which is experienced as minimally offending the personal integrity.
The device according to U.S. Pat. No. 5,458,853 is proposed in the first place for the police department""s xe2x80x9cflyingxe2x80x9d control of the soberness of drivers and aims at reducing, for the purpose of saving costs, the use of mouthpieces but there are many other situations wherein it is desired to automatically and rapidly effect unattended alcohol tests in a xe2x80x9cpassivexe2x80x9d manner for example alcohol tests on visitors to a public entertainment when the visitors pass an entrance, alcohol tests on drivers at payment sites on toll roads and at garage exits, and alcohol tests on employees at working places where alcohol problems exist among the personnel, i.e. situations where it is desired to screen off more or less drunken persons. Another example of a passive alcohol test is when a person serves a sentence at home provided with an electronic foot shackle in order to check that the sentenced person satisfies the requirement of refraining from alcohol.
xe2x80x9cAmerican J. Public Health, volume 83(4), pages 556-560, (1993)xe2x80x9d discloses the use of the CMI/MPH Alcolmeter VAS in field studies for passive alcohol testing. However, the design of the said meter is not disclosed therein.
xe2x80x9cClinical Science, volume 63, pages 441-445, (1982)xe2x80x9d discloses that there is a relation between temperature, humidity and alcohol concentration in exhaled air from persons with alcohol in the blood. The alcohol concentration was determined by a gas chromatograph attached to a mouthpiece.
The object of the invention is to make possible that the concentration of a specific substance in the blood is indirectly determined by xe2x80x9cpassivexe2x80x9d measurement of the concentration of said substance in a person""s exhalation air with greater accuracy than can be achieved in prior art methods and devices by minimizing the error which is dependent on variation of the evaporation temperature (lung temperature) and even with greater accuracy than can be achieved by active devices wherein exhalation takes place through a mouthpiece and a special breathing technique and the tested person""s complete participation is required.
A further object of the invention is to make possible that the measurement is performed rapidly by the tested person exhaling freely without special breathing technique being required for performing a reliable measurement.
In order to achieve these objects it is proposed according to the invention a method for determining indirectly the concentration of a specific substance in the blood of the kind referred to above having the characterizing features of claim 1.
The invention also provides an apparatus for working the method according to claim 11.
The concentration of alcohol in the exhalation air is dependent not only of the alcohol concentration in the 30 blood but alsoxe2x80x94in the same manner as the concentration of water vapourxe2x80x94of the lung temperature. The change of the saturation pressure, in dependence of the temperature is different for water and alcohol, respectively, but the difference is small. If 37xc2x0 C. is chosen as a normal temperature the error in the measurement when applying the method according to the invention will be a deviation of about 0.9%/xc2x0 C. from the normal temperature, which should be compared with the accuracy of about 7%/xc2x0 C. achieved when applying prior art technique.
In order to eliminate also this error in the measurement of about 0.9%/xc2x0 C. which is low per se there is proposed according to claim 20 also a modification of the method according to the invention wherein the test person exhales through a mouthpiece at one end of a defined flow passage in order that the concentration of said specific substance and the concentration of water vapour shall be determined by measuring the undiluted alveolar gas, and the deviation of the evaporation temperature from a normal temperature can be determined for correction in determining the content of the specific substance in the blood.